This invention relates to compositions containing photosynthetic microorganisms, preferably algae, for application to soil to provide a cover crop and to improve soil aggregation.
Cover crops are planted to improve the soil by adding organic matter, enhancing soil aggregation, controlling erosion, and improving moisture retention. They serve to improve the soil for the benefit of the cash crop. An example is red clover.
When plant material from cover crops decomposes, organic matter is added to the soil. When planted for this purpose, cover crops are known as green manure. Green manuring has been practiced in different forms, such as growing a legume crop in situ before planting the cash crop, simultaneous cultivation of a green manure and the cash crop, and addition of leaves and cuttings of plants or trees that have been grown elsewhere before or after planting the cash crop. Benefits credited to green manuring include increases in available plant nutrients and organic matter content and improvement in the microbiological and physical properties of the soil.
Traditional cover crops, however, have certain drawbacks that can outweigh their advantages. They can compete with the cash crop for moisture, nutrients, or sunlight, if grown with the cash crop. If grown in lieu of the cash crop, valuable land is taken out of production and the revenues that would results from the land being planted with the cash crop are lost. In addition, to provide the benefits of green manure, the conventional cover crop must be plowed under resulting in the farmer's expenditure of time and money for such things as fuel.
To overcome the disadvantages of traditional cover crops, microbial cover crops have been used. Certain photosynthetic microorganisms, especially algae, have been found to help control erosion, improve moisture retention, enhance soil aggregation, provide nutrients and organic matter to the soil, and reduce salinity. Algae cultures can be applied to the soil easily, for example, by mixing with water and spraying the suspension on the soil. Algae can also be grown at the same time as the cash crop with little competition for nutrients, space, sunlight, or moisture. The time for "planting" the algae to maturity, when it provides most of its benefits, is a matter of weeks rather than the months required to grow traditional cover crops. In addition, there is no need to plow the algae under for it to serve as a green manure.
Algae in the soil surface layers function autotrophically as green plants, utilizing their chlorophyll to convert carbon dioxide, nutrients, and inorganic nitrogen into cell substance by means of energy derived from sunlight. Soil algae are divided into Chlorophyta or green algae, Cyanophyta or blue-green algae, Bacillariophyta or diatoms, and Xanthophyta or yellow-green algae. Blue-green and green algae are the subject of the preferred embodiments of this invention. Blue-green algae are procaryotic, and many, but not all, species fix nitrogen. Green algae are eucaryotic and do not fix nitrogen. Some species of green or blue-green algae are unicellular and others are filamentous. Although algae usually reproduce asexually by cell fission, some types can also reproduce sexually.
It is known that many species of algae, particularly those of the families Chlorophyta and Cyanophyta, can survive dessication, sometimes for many years. Trainor, F. R., Phycologia, 9:111-113 (1970). Some species of algae, such as Chlamydomonas mexicana, under sexual reproduction when nutrients in the environment are depleted, which results in a cell type known as a zygospore that is very resistant to dessication. Lewin, R. A., J. Gen. Microbiol., 5:926-929 (1951).
One of the important benefits of growing algae on soil is the modification of soil structure through the aggregation of soil particles. Bailey D., et al. J. Phycol., 9:99-101 (1973), incorporated herein by reference. Improved soil aggregation leads to better root growth, better transport of nutrients, water, and gases in the root zone, decreased erosion, and better water retention. Preliminary observations of irrigated soil innoculated with Chlamydomonas mexicana indicated improved penetration and percolation of water, favorable crop response, and reduced erosion. Metting , B. et al., Soil Science Soc. of Amer. J., 47:682-685 (1983), incorporated herein by reference.
Soil aggregation is caused by a mucus-like material excreted by the algae that binds soil particles. The material is believed to be composed primarily of polysaccharides, which are known to be flocculants. Little work has been done to characterize in detail these polysaccharides and any other substances that may be in the material. Thus, this application uses the terms "flocculant" and "flocculants" to characterize the soil aggregating material produced by algae and other photosynthetic microorganisms.
The use of certain types of algae in a method for treating soil to promote soil particle aggregation is disclosed in U.S. Pat. No. 3,969,844 to Fogel et al., issued July 20, 1976. Fogel et al. discloses a method comprising applying flocculant-producing algae to soil, culturing the algae under conditions favoring cell multiplication on the soil until a desired population density is reached, and thereafter continuing to culture the algae under conditions that favor the production of flocculants for a time sufficient to achieve the desired soil aggregating properties. In particular, certain algae, preferably those of the genus Chlamydomonas, especially the species Chlamydomonas mexicana, are cultured in nurse pools on a liquid nutrient medium under conditions that produce logarithmic or exponential growth. The cultures are harvested as an aqueous suspension which is then transported to the field and uniformly distributed on the soil to be treated. Nitrogen, other nutrients, and moisture are added or maintained in sufficient quantities so that the algae continue to multiply on the soil until a predetermined population density is reached. Then, the nutrients other than nitrogen are maintained, causing nitrogen to be depleted as the algae continue to grow. Thus, a state of nitrogen deficiency is created. This nitrogen deficient state, according to Fogel et al., promotes the production of the flocculants. Thus, Fogel et al. teaches that the creation of a nitrogen deficient state is critical to the production of algal flocculants.
The critical nature of the nitrogen deprivation step for enhancing the production of algal flocculants is also taught by U.S. Pat. No. 3,958,364 to Schenck et al., issued May 25, 1976. Schenck et al. discloses methods for the cultivation of algae for the production of flocculants. Preferred algae disclosed by Schenck et al. are those from the genus Chlamydomonas, preferably the species Chlamydomonas mexicana, and from the genus Chlorella, preferably the species Chlorella pyrenoidosa.
The soil treatment method of Fogel et al. presents significant disadvantages when actually used in the field. With respect to the nitrogen depletion step, there are a number of problems. Having to add nitrogen through the application of fertilizer for a particular period of time and then allowing a state of nitrogen deficiency to be created in time-consuming and not cost-effective for the farmer. Also, it is very difficult to monitor and control with available farm machinery the amount of available nitrogen in the top 1 to 2 centimeters of the soil, where the algae are concentrated.
Maintaining and transporting algae in the vegetative or growing stage also presents several problems. Contamination of the culture by undesired microorganisms is always possible when a large-volume liquid culture of algae is to be maintained. Furthermore, in order for the algae to remain viable, it is necessary for the nutrient supply to be constantly monitored and replenished as the culture grows. The cell population density of the culture must also be watched to assure that the algae are receiving adequate light. The addition of air and carbon dioxide is also required for long-term maintenance. Moreover, the storage and transportation of the large quantities of algae required for use as a soil conditioner is made more difficult and costly by the bulk of the liquid culture. At least half of the cost of transporting the product is attributable to this bulk and weight. Further, the maximum shelf life of such a product--the time before significant loss of viability occurs--is approximately 14 days. The high transportation costs and limited shelf life tend to limit the scope of practical geographical distribution of the product. In particular, it is not commercially feasible to export the product.
In addition to being used to improve soil aggregation, algae have been used to fix nitrogen. In particular, certain blue-green algae have been used in flooded rice fields for this purpose. It is well established that the rice field ecosystem is a favorable environment for the growth of blue-green algae and that nitrogen fixation by blue-green algae plays a vital role in the buildup and maintenance of soil fertility in such fields. Release of nutrients through microbial decomposition after the death of the algae appears to be the principle means by which nitrogen is made available to the rice. Roger, P. A. and Kulasooriya, S. A., Blue-Green Algae and Rice (Manila: The International Rice Research Institute, 1980), pgs. 49-50, hereinafter Roger and Kulasooriya. The entire focus of Roger and Kulasooriya is on the use of blue-green algae for nitrogen fixation in flooded rice fields. In fact, they report that the use of the algae seems to have little effect on the physical properties of the soil, although it may improve soil aggregation. Ibid., p. 77.
Roger and Kulasooriya disclose two methods for the production of a dry form of blue-green algae for application to flooded rice fields. One method comprises growing the blue-green algae in liquid culture, mixing the algae with an inert material for support, and drying the mixture. From the information provided, the supports appear to be incapable of dispersion in water or uniform application to soil. One such support is sand. Ibid. pgs. 81-85. The other method comprises growing blue-green algae in trays of liquid culture, allowing the water in the culture to evaporate in the sun to produce dry flakes or a dry mass, and collecting the flakes or scraping the mass off the bottom of the trays. Ibid., pg. 82.
There are a number of disadvantages to the methods disclosed by Roger and Kulasooriya. First, they have been shown to be applicable only to fields submerged under water; i.e., rice fields. Uniform application of the algae to the field is attained by introducing the algae non-uniformly into the rice field followed by vegetative propagation of the algae and uniform dispersion over the field by virtue of the motility of the algae in the water covering the field. Virtually any cash crop of interest other than rice is grown on a dry field.
Second, the material produced is not capable of an economical, uniform dispersion on a large area of soil using modern agricultural equipment, as is the case with most commercial farming in developed countries. For dried algae produced by the first method, the inert materials supporting the algae, such as sand, are not adapted to easy, uniform application to a field. Rather, the technique appears to be designed to use labor intensive methods, in particular, dispersal by hand. In fact, the authors state that a disadvantage of using sand is that the heavy particles sink into the mud and therefore hamper the rapid growth of the adhering algae. Ibid., p. 82. Dried algae produced by the second technique would also not be capable of economical, uniform dispersion on a large, dry soil surface using modern agricultural equipment. Because of the filamentous nature of most blue-green algae, the dried mass cannot readily be reduced to small particles or suspended in water.
Applicants have surprisingly discovered compositions of flocculant-producing photosynthetic microorganisms and methods for preparing and using the compositions to produce microbial cover crops which overcome the disadvantages of the above-mentioned compositions and methods. Applicants have invented compositions containing drought-resistant, dormant, flocculant-producing photosynthetic microorganisms that can be easily and cheaply transported virtually any distance and are capable of easy, uniform application to a dry field, such as by forming a suspension in water and spraying the suspension on the field. As a result, the cost of transporting such compositions is expected to be cut at least in half and possibly by as much as a factor of 10, and the shelf life of such compositions is expected to increase from two weeks to at least six months and perhaps as much as 18 months. Applicants have also invented methods of preparing such compositions and of using them to provide a cover crop and to improve soil aggregation. Finally, applicants have discovered, contrary to the teachings of the art, that it is not necessary for certain algae in the vegetative stage to be deprived of nitrogen before being able to produce significant quantities of flocculants.